1. Field of the Invention
The present invention relates generally to instruments that measure physiological parameters and in particular to electronic instruments of the kind that generate digital signals indicative of physiological parameters to drive monitors that can generate visual displays representative of those parameters.
2. The Prior Art
Modern medicine employs electronic instruments to measure various physiological parameters such as body temperature or the pressure of various bodily fluids. One example of such an instrument is an electronic blood pressure meter for measuring blood pressure at a point within a human body. Such a blood pressure meter employs a transducer having a flexible member mechanically connected to one end of a catheter containing an inert fluid. The other end of the catheter is passed through a blood vessel in the body to the desired measurement point. Pressure exerted by the blood is transmitted through the inert fluid back to the flexible member, causing the flexible member to deform. As the flexible member deforms, a resistance strain gage, mechanically coupled to the flexible member, changes resistance in proportion to the magnitude of the pressure. The strain gage is wired as a leg of a Wheatstone bridge that can be electrically connected to an external blood pressure monitor for generating a visual display indicative of the blood pressure.
The external blood pressure monitor provides an excitation signal to the bridge and receives therefrom a response signal having a magnitude proportional to the blood pressure. Because the bridge is a simple resistance bridge, the excitation signal can have any desired magnitude up to several volts and any desired frequency from DC to several hundred kilohertz, and in practice excitation signals having various magnitudes and frequencies are provided by different blood pressure monitors on the market. A given monitor generates an accurate readout only if the bridge to which the monitor is connected has been excited by an excitation signal having the same magnitude and frequency as the excitation signal provided by that particular monitor.
Recent years have seen the development of digital electronic blood pressure meters having various desirable features not found in conventional electronic blood pressure meters of the kind described above. Such digital meters do not employ resistance strain gages and hence cannot be used with conventional external blood pressure monitors. Although it is possible to design new monitors to provide blood pressure readouts in response to digital output signals from such digital meters, presently there is no way a conventional monitor can generate a readout from such a signal. Conventional monitors are in widespread use in medical facilities, and it would be highly beneficial to be able to use these existing monitors in conjunction with the new digital blood pressure meters. Instruments that generate digital signals indicative of physiological parameters other than blood pressure are also finding increased use in the practice of medicine, and it would be advantageous to be able to use conventional monitors with these instruments as well as with the new digital blood pressure meters. Accordingly there is a need for a way to adapt a digital physiological measuring instrument to drive a conventional monitor to generate a visual display. The present invention satisfies this need.